"People around the league, they appreciate, they understand how special it is," Horford said Friday at a news conference surrounded by championship banners at the Celtics' practice facility. "I hope that if I'm coming here, it will definitely open the door to other free agents."

Boston Celtics forward Al Horford walks past a blank NBA Championship banner while arriving for a media availability at the team's practice facility, Friday, July 8, 2016, in Waltham, Mass. Horford agreed to a four-year, $113 million deal with the Celtics as an unrestricted free agent, ending nearly ten years with the Atlanta Hawks. (AP Photo/Charles Krupa)

Horford agreed to a four-year, $113 million deal with the Celtics last week on the opening day of NBA free agency. A few hours later, he learned that Kevin Durant had spurned Boston's offer and decided to sign with the Golden State Warriors instead.

"It would have been nice, but I have to move on," Horford told reporters Friday morning, after he was introduced but before the contract had been approved by the NBA office.

Without Durant, the Celtics missed out on a player that could have elevated the team into immediate contention for the franchise's 18th NBA title. General manager Danny Ainge conceded that Horford alone doesn't do that, even on a roster that already included All-Star Isaiah Thomas.

"Right now it definitely makes us better," Ainge said. "We're not done. We still have work to do."

The news conference was delayed at the start because of a paperwork problem that kept the Celtics from saying Horford was officially signed. Owner Wyc Grousbeck said it was just a formality, but in the meantime they were only "welcoming" Horford to Boston.

That didn't stop the player's wife from taking a panoramic photo of the 17 NBA championship banners ringing the practice court. The numbers on the scoreboards had all been changed to "18" — the next Celtics title.

"He might not have seen it, there's a blank banner up there on the right," Grousbeck said. "We're hoping to work together on that."

The Celtics have all those banners and retired numbers, but NBA free agents have traditionally been drawn more to the attractions of South Beach or the media possibilities in New York and Los Angeles. Boston has signed some big-name free agents in the past — Shaquille O'Neal and Dominique Wilkins among them — but only at the tail end of their careers.

During Ainge's last roster rebuild, Timberwolves big man Kevin Garnett balked at coming to Boston — in a trade — until Ray Allen was acquired — also in a trade — and added to a roster that already had Paul Pierce. Now that the Celtics have Horford, they could be a more attractive destination for the next crop of free agents.

"It's a great organization. There's a lot of history here," Horford said. "I feel like everyone around the league respects it. That's a big part of the reason why I chose to come here.

"This is a special place. People are open to it," he said. "They will see how special it is."

A 6-foot-10 center who spent his first nine years with the Hawks, Horford has averaged 14.3 points and 8.9 rebounds in his career. And he saw firsthand what the Celtics were building in the playoffs this season, when Atlanta eliminated Boston in six games.

"I'm very encouraged by the group of players that are here," he said. "Young, talented hardworking players. Playing in that series, I saw the passion that they played with and how much they had gotten better during the season."

The feeling is mutual.

"The way that he was flying around in that playoff series, and the harm's way that he put us in, it's great to have him here now," Celtics coach Brad Stevens said.

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Follow Jimmy Golen on Twitter at http://www.twitter.com/jgolen .

Boston Celtics forward Al Horford holds up a jersey during a media availability at the team's practice facility, Friday, July 8, 2016, in Waltham, Mass. Horford agreed to a four-year, $113 million deal with the Celtics as an unrestricted free agent, ending nearly ten years with the Atlanta Hawks. (AP Photo/Charles Krupa)

Boston Celtics forward Al Horford walks past NBA Championship banners while arriving for a media availability at the team's practice facility, Friday, July 8, 2016, in Waltham, Mass. Horford agreed to a four-year, $113 million deal with the Celtics as an unrestricted free agent, ending nearly ten years with the Atlanta Hawks. (AP Photo/Charles Krupa)

Boston Celtics forward Al Horford, second from left, smiles during a media availability at the team's practice facility, Friday, July 8, 2016, in Waltham, Mass. Horford agreed to a four-year, $113 million deal with the Celtics as an unrestricted free agent, ending nearly ten years with the Atlanta Hawks. From left are Celtics owner Wyc Grousbeck, Horford, general manager Danny Ainge and head coach Brad Stevens. (AP Photo/Charles Krupa)

Boston Celtics forward Al Horford holds up a jersey during a media availability at the team's practice facility, Friday, July 8, 2016, in Waltham, Mass. Horford agreed to a four-year, $113 million deal with the Celtics as an unrestricted free agent, ending nearly ten years with the Atlanta Hawks. (AP Photo/Charles Krupa)

Boston Celtics forward Al Horford smiles during a media availability at the team's practice facility, Friday, July 8, 2016, in Waltham, Mass. Horford agreed to a four-year, $113 million deal with the Celtics as an unrestricted free agent, ending nearly ten years with the Atlanta Hawks. (AP Photo/Charles Krupa)